Packaging machines for the wrapping of packing pieces with plastic film are often used for the packing of newspapers and magazines. The newspapers and magazines are typically guided through the transport channel by means of the transport device in packs of different heights or as a single copy.
The plastic film is generally pulled from two rolls, one of which is arranged below the transport device, and the other one above. The plastic films are welded or connected approximately at the height of the transport device. As a result, the plastic films form a closed web or film wall running transversely to the transport direction of the packing pieces (e.g., newspapers and magazines). The plastic film from the lower roll applies itself from below and between the transport device as the packing pieces move against the plastic film from the lower roll. The upper roller guides the plastic film to be deposited around the front face and the upper side of the packing pieces. The wrapped packing pieces are then guided to the transport device with the help of the film cutting and welding or connecting unit descending from above in the direction of the transport band. The plastic film from the upper roller is welded or connected to the plastic film from the lower roller by two parallel weld seams and the plastic films are then cut between the two weld seams. The packing piece which is wrapped in such a manner with a circulating film tube at its front and back face, and its upper and lower side, is subsequently ejected by the transport device.
While the plastic film is pulled comparatively slowly at its front face and upper side during the wrapping of the packing piece from above with only a little more than the transport velocity of the packing piece, it experiences a large acceleration when the film cutting and welding unit is actuated for closing of the film tube at the backwardly arranged face and takes along the film supplied from above during its fast downward movement in the direction of the transport band. During the comparatively sudden standstill of the film cutting and welding unit, the film pulled from above from the pulling unit tends to run-up due to its inherent momentum and tends to deposit itself in the transport channel for the packing pieces in waves of excess material instead, as desired, to stand as a largely flat vertical film wall after the welding to the packing piece which is supplied from below. The transport channel is thereby blocked very quickly and operation disruptions result, so that an orderly wrapping of the packing pieces cannot be ensured and a frequent standstill of the machine results.
In the past, the problem of the undesired film run-on was met by comparatively elaborate mechanical devices which were controlled in dependence of the movement of the film cutting and welding unit so as to decelerate the film synchronously with the film cutting and welding unit. It has for example been suggested to provide a brush ledge which can be lined up against the film and to press it against a support, which ledge is arranged in the machine flame in a pivoted manner and which is pressed against the film by a drive, when the film cutting and welding unit decelerates during the approach to its lowest position, so that the momentum inherent to the film is cancelled by this friction brake. This arrangement is comparatively elaborate and error-prone.
In view of the present state of technology with respect to wrapping machines, there is a demand for a plastic wrapping machine that reliably and efficiently wraps packing pieces and minimizes waste of plastic film during the wrapping process without the use of elaborate and/or complex mechanical arrangements.